Vital signs are important indicators of a person's health and well-being as well as predictors of acute medical conditions and chronic disease states for a person. Breathing rate is one of the most important vital signs, which can be measured by the number of exhalation and inhalation a person takes per minute. In addition, the breathing pattern may be highly correlated to psychological conditions of a human being, such as stress and anxiety.
Many important human vital signs such as breathing are periodic motions. Most traditional approaches for breathing monitoring are invasive in that they need physical contact of the human bodies. For instance, in hospitals, the patients are required to wear oxygen masks, Nasal cannulas, chest straps, or wearable sensors such as thermistors and pressure sensors. Another example is Polysomnography (PSG) used in sleep medicine, which typically requires a minimum of 22 wire attachments to the patient. These dedicated devices are often costly and bulky, create discomfort to the human bodies, and are limited only to clinical settings. Although these wired medical systems can measure breathing using wearables attached onto human body, such systems are clumsy and uncomfortable. To make it worse, the systems themselves would itself distort the very breathing that need to be monitored—as many patients become anxious or annoyed with all the attached wearables and wires.
Currently existing non-invasive (contact-free) breathing monitoring solutions can be categorized as below.
(1) Radar-based breathing monitoring: Doppler radars are often used in breathing monitoring. They are operated by transmitting a signal and receiving a signal with a Doppler shift due to a periodic motion of objects. The breathing rates can be extracted from the Doppler shift. As a drawback, these systems use high transmission power, rely on sophisticated and expensive hardware, and use extremely large bandwidths. A vital sign monitoring system was disclosed utilizing frequency modulated continuous radar (FMCW). It used Universal Software Radio Peripheral (USRP) as the RF front-end to transmit a frequency-sweeping signal. But the additional cost and complexity of the dedicated hardware limited a large-scale deployment of FMCW radar.
(2) Wireless-sensor based breathing monitoring: The received signal strength (RSS) measurements from 802.15.4 compliant sensors on multiple 802.15.4 channels were also used for breathing detection and breathing rate estimation. Dense deployment of wireless sensors is required in these methods as additional wireless infrastructures. In addition, the specific design of frequency-hopping mechanism is required to support multiple channel measurements.
(3) Wi-Fi-based breathing monitoring: RSS is commonly used in the Wi-Fi-based breathing monitoring due to its availability on most commercial Wi-Fi network interface controllers (NICs). Measurements were also used with Wi-Fi devices for breathing estimation. But this method is accurate only when the users hold the Wi-Fi-enabled devices in close proximity to their chests.
In addition to the drawbacks mentioned above, methods (1) and (2) require design and manufacturing of special devices such as specialized radar devices or sensor network nodes, while method (3) has very low accuracy and sensitivity. A wireless breathing monitoring system was proposed to monitor breathing rate and heart rate based on UWB (ultra-wide band) signal. But the UWB-based system has many limitations such as: it has expensive, untested, uncommon, dedicated, limited edition experimental hardware components; it has a small range due to severe absorption of UWB signals by walls; it works only in a line-of-sight (LOS) condition; and it needs very tedious and laborious deployment of many sensors in practical situations to cover a reasonable area, making installation, maintenance and repair very expensive and labor intensive due to the restrictive LOS operation.
Therefore, there is a need for methods and apparatus for vital sign detection and monitoring to solve the above-mentioned problems and to avoid the above-mentioned drawbacks.